1. Field of the Invention
The present invention relates generally to the control of electric machines. The present invention relates more specifically to the control of electric machines through the digital selection of operating states.
2. Discussion of the Related Art
Advanced electric motor drives are continually gaining popularity in motion control applications, with major users including consumer appliances, industrial machinery, aerospace applications, and automotive applications. Motor drives are used in a broad variety of applications from low-power home appliances such as washing machines, refrigerators, air conditioning, hand power, tools, and cordless drives, robots, fitness machines, and medical instrumentation, to medium-power automotive applications such as electric power steering, active suspension, brake by wire, starter/alternator, and anti-lock braking systems, to high-power industrial motor drives and automation systems, electric and hybrid electric cars, propulsion systems for trains and locomotives, mass transit, movers, machine tools, elevators, pumps, and compressors. For all these applications, it can be desirable to have a low cost, but effective motor speed controller.
Hysteresis (band or bang bang) current control and PWM control are the most widely used speed control techniques for electric motors. Both of these control techniques are analog based but can be easily implemented on analog or digital components since they are well understood. Cost and implementation complexity are often the most important factors for design trade-offs between techniques, implementation, and strategy of motor control hardware. For digital implementation of the aforementioned techniques, microcontrollers, microprocessors, or digital signal processors (DSPs) may be used. However, it is important to note that digital implementation of such a control technique does not produce a true digital controller. Instead, what results is a digitally implemented non-digital control technique.
Commonly assigned U.S. Pat. No. 7,193,385 provides a digital controller for electric machines in which a binary state digital controller deals with the motor drive as a binary state digital system. The controller desirably operates with a binary choice of predefined motor operation states to control speed or torque of the motor, digitally selected by simple IF/THEN logic based upon greater than/less than comparisons of a single operating motor value to a single control parameter.